The advantages of permeable hollow fibers as separatory membranes are now well known. For example, the large membrane areas per unit volume of device attained with this membrane configuration are of particular importance in minimizing priming blood requirements for devices such as so-called artificial kidney or artificial lung units. Additionally, a simplification over flat membrane devices is realized in that hollow fibers, by reason of their cross-sectional shape, are essentially self-supporting and do not collapse under the transmembrane pressures required for processes such as reverse osmosis of sea water.
Polyethylene is a particularly attractive material for hollow fiber manufacture because it is inexpensive, relatively inert, non-toxic, readily processed and strong. Polyethylene hollow fibers would be expected to lack the hydrophilicity which is characteristic of the types of membranes which have been most widely used; i.e., in such processes as recovery of water from brines by reverse osmosis. However, hydrophilicity is not essential for permeability separations such as removal of dissolved gases from aqueous or non-aqueous solutions or ultrafiltration of organic solutions containing relatively large solute molecules.
The only process known to Applicants for making polyolefin hollow fibers is that disclosed in U.S. Pat. No. 3,423,491, which teaches the preparation of permselective hollow fibers by melt spinning a mixture of a thermoplastic polymer and a plasticizer, cooling and leaching out the plasticizer. The following plasticizers, disclosed as leachable with alcohols or aromatics, are listed as suitable for the fabrication of semi-permeable hollow fibers from polyolefins: dioctyl phthalate, polyethylene wax, tetrahydronapthalene and chlorinated biphenyls.
It is apparent from the data give in the U.S. Pat. No. 3,423,491 (for cellulose triacetate fibers) that the membrane structures obtained by the disclosed process are much "tighter" than is required for the types of separations for which polyolefin hollow fibers appear to be most suitable. A microporous structure, in which selectivity results primarily from pore size rather than from the chemical nature of the membrane material, would appear to be preferable.
U.S. Pat. No. 3,745,202 is directed to the preparation of porous hollow fibers having a semipermeable outer layer or "skin". A mixture of a cellulose ester or ether with a plasticizer is melt spun and the resulting molten fiber is drawn, gelled by cooling and leached to remove the plasticizer. At this stage, the fiber structure is said to have a graded porosity but to lack the outer skin required for selectivity. After a following treatment with hot water, the fibers are said to be more crystaline and more permeable but require further treatment for attainment of selectivity.
U.S. Pat. No. 3,093,612 (corrected to 3,092,612) is directed to polyolefin/alkoxyalkyl ester compositions which are suitable for the preparation of solid polyolefin filaments comprising "small pores" which are not contiguous. A polyolefin/ester mixture is heated to form a solution, forced through a spinnerette, solidified by cooling and immersed in a wash bath, such as isopropanol. The resultant fiber is air dried and then stretched (drawn) to achieve a maximum degree of orientation. Most of the solvent (ester) is said to be removed during coagulation of the fiber and the rest (except for a residual solvent content) by washing.